Method for producing components with a high load capacity from TiAl alloys

ABSTRACT

The invention relates to a method for producing components with a high load capacity from α+γ TiAl alloys, especially for producing components for aircraft engines or stationary gas turbines. According to this method, enclosed TiAl blanks of globular structure are preformed by isothermal primary forming in the α+γ− or α phase area. The preforms are then shaped out into components with a predeterminable contour by means of at least one isothermal secondary forming process, with dynamic recrystallization in the α+γ− or α phase area. The microstructure is adjusted by solution annealing the components in the α phase area and then cooling them off rapidly.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing heavy-duty componentsfrom α+γ TiAl alloys, especially components for aircraft engines orstationary gas turbines.

TiAl-based alloys belong to the group of intermetallic materials, whichwere developed for uses at temperatures at which super alloys are used.With a density of about 4 g/cc, this new class of alloys offers aconsiderable potential for weight reduction and, in associationtherewith, a reduction in stresses of moving components at temperaturesup to above 700° C. This weight and stress reduction acts exponentiallyalso on the buckets and blades of gas turbines or, for example, ofcomponents of piston engines. The difficulty of processing TiAl alloysby shaping processes is based on the high yield points as well as thelow fracture toughness and ductility at low and moderate temperatures.Shaping processes must therefore be carried out at high temperatures inthe region of the α+γ or α phase areas under an inert atmosphere.

U.S. Pat. No. 6,110,302 discloses α+γ titanium alloys. Among otherthings, turbine blades for aircraft engines are dealt with. The use ofalloys with about 70% titanium is preferred, the forging temperaturebeing between 815° C. and 885° C. The forging, forming such products asturbine blades, is to have β+α−β regions of different microstructure.Practical investigations have shown that turbine blades, producedaccording to this method, do not satisfy the requirements in theoperating state, especially with regard to the desired fatigue strength.

U.S. Pat. No. 5,593,282 discloses a rotor, which can be used in enginesand may be formed, preferably, from a lightweight construction material,in this example from a temperature-resistant ceramic material or,alternatively, from TiAl or NiAl materials.

In the DE-C 43 18 424, a method is described for producing moldedobjects from alloys based on titanium and aluminum. A cast preform witha lamellar structure with a thickness of up to 1 μm is produced. This isshaped at a temperature ranging from 1050° C. to 1300° C. with a highdegree of deformability, so that a dynamic recrystallization withparticle sizes up to 5/μm takes place. Subsequently, the preform iscooled and shaped superplastically at temperatures ranging from 900° C.to 1100° C. at rates of 10⁻⁴/s to 10⁻³/s to molded objects having almostthe final dimensions. The very fine-grained structure addressed isproduced, for example, by the addition of up to 0.3% by weight ofsilicon. However, this proportion of silicon leads to undesirable sideeffects, such as an increased porosity and the formation of silicides,as a result of which the mechanical stressability is affected greatly.The fine-grained structure, required for this superplastic shaping is tobe brought about by extrusion molding, which does not, however, lead tothe finely crystalline, equiaxial structure, which is describedelsewhere and required for the superplastic shaping. The extent, towhich components, which can be stressed highly mechanically, canactually be produced by this method, is unknown, since this method hasnot yet gained acceptance in practice.

On the basis of the shaping factors, shown here, the manufacturingmethods, addressed in the state of the art and intended, for instance,for TiAl components, do not lead to the technical quality propertiesrequired for components, which can be highly stressed dynamically andthermally.

SUMMARY OF THE INVENTION

Starting out from the disadvantages listed in the state of the art, itis an object of the invention to make available a method for theproduction of light-weight, heavy duty components for the conventionaltechnology and air traffic technology from TiAl alloys with which, incomparison to state of the art, improved fatigue strength, reliabilityand an increased service life can be realized.

This objective is accompanied by a method for the production of heavyduty components from α+γ TiAl alloys, especially of components foraircraft engines or stationary gas turbines, in that encapsulated TiAlpreforms of globular structure are pre-shaped by isothermal primaryshaping in the α+γ or α phase area, the pre-shaped preforms are shapedby at least one isothermal secondary process with dynamicrecrystallization in the α+γ or α phase area to components of aspecifiable contour and, for setting the micro structure, the componentsare solution annealed in the α phase area and subsequently cooledrapidly.

Advantageous further developments of the inventive method may beinferred from the dependent claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Deviating from the state of the art of U.S. Pat. No. 6,110,302 and DE-C43 18 424, TiAl preforms are now shaped repeatedly at temperature rangesabove the temperatures given there and achieve structure properties,which are associated with a longer service life than that of the stateof the art. Moreover, the use properties, especially the fatiguestrength, can be improved significantly.

Very homogeneous, TiAl preforms are used with a globular grainstructure, which is subjected in an appropriate manner to a primaryshaping, which is followed by a secondary shaping, in the α+γ or α phasearea.

The primary shaping can be accomplished by forging or extrusion molding.The secondary shaping advantage is accomplished by forging.

During the primary shaping, as well as during the secondary shaping, theforging preforms are encapsulated, for example, by a shape-producingtool with an upper and a lower part, as is understood by those skilledin the art.

Contrary to the state of the art of DE-C 43 18 424 (process window ofsuperplasticity), the suitable forging windows are characterized by apronounced flow-stress maximum. The dynamic recrystallization, which isassociated with a high yield point, is characteristic of the inventiveshaping process. The microstructure is made available by solutionannealing of the components in the α phase area and subsequently,cooling them rapidly. This rapid cooling from the α phase area thenleads to the desired fine lamellar microstructure. Typical cooling ratesfor this purpose are of the order of 10° C./s.

Advantageously, for producing the lightweight, heavy duty components forconventional technology and air traffic technology, preforms of thefollowing composition (in atom percent) are used:

-   43%–47%, especially 45%–47% Al-   5%–10% Nb-   maximum 8.0% B-   maximum 0.5% C-   Remainder: titanium and impurities resulting from the smelting.

Silicon is not contained in these alloys. Although, on the one hand, asis well known, it contributes-to--the desired grain refining, it also,on the other, leads to the already addressed, undesirable side effects,such as porosity and silicide formation.

The isothermal shaping (primary and/or secondary) advantageously takesplace in heated tools of molybdenum or graphite.

The following example describes a method for producing rotor disks,which may be used in aircraft gas turbines. The example may also referto heavy-duty components, other than those for conventional technologyor air traffic technology, such as components of internal combustionengines, such as valves.

A preform of the following chemical composition (in atom percent) isused:

-   46% Al-   7.5% Nb-   0.3% C-   0.5% B-   remainder: Ti

In a first step, the preform is subjected to an isothermal primaryshaping at an α+γ temperature of 1200° C. A flat track-forging die isused, with which so-called pancakes are produced. The isothermal primaryshaping takes place at a rate of 10⁻⁴/s. In a second, isothermal forgingprocess, the pancakes are forged into finished disks with ashape-producing forging tool with an upper part and a lower part. Inthis example, the isothermal secondary shaping takes place at an α+γtemperature of 1150° C. and a shaping rate of 10⁻³/s.

For adjusting the later use properties of the rotor disks, so produced,the latter are solution annealed at a temperature of 1360° C. andsubsequently cooled rapidly in oil at a rate of 10° C./s. The finishingis conventional and not an object of this invention.

The following example shows a method of producing turbine buckets, whichcan be used in stationary gas turbines.

A preform of the following composition (in atom percent) is used:

-   45% Al-   8% Nb-   0.2% C-   Remainder Ti

The first forging process of a basic material for α+γ TiAl preformstakes place in this example owing to the fact that the volumedistribution for a larger number of preforms (10 here) is carried out inthe α+γ phase area at about 1150° C. in a forging die with a disk-shapedrecess. In this example, the preforms are segregated at a hightemperature by a cutting tool. As a result, cooling of the preforms withsubsequent re-heating for further shaping processes becomes unnecessary.

In a second isothermal forging process, the forging of the preform tobuckets is completed in a shaping forging tool with an upper part and alower part. The secondary shaping takes place in this example in the α+γphase area at about 1150° C. and at a shaping rate of 10⁻³/s.

To set the later use properties of the turbine buckets so produced, thelatter are solution annealed at an α temperature of 1360° C. andsubsequently cooled rapidly in oil.

The production processes of further components differ from this exampleonly in their geometric formation.

The composition of the alloys, described above, as well as thetemperature ranges selected for the primary and secondary isothermalshaping merely represents examples.

1. A method for producing heavy-duty components from α+γ TiAl alloys foraircraft engines or stationary gas turbines, in that preshapingencapsulated TiAl preforms of globular structure by isothermal primaryshaping in the α+γ or α phase area in a temperature region of 1000° C.to 1,340° C. by forging or extrusion molding; shaping the preforms byforging to form components with a specifiable contour by at least oneisothermal secondary shaping process simultaneously with dynamicrecrystallization in the α+γ or α phase area in the temperature regionof 1000° C. to 1,340° C. in an inert atmosphere, said preshaping andshaping steps be carried out in a heated tool of molybdenum or graphite;adjusting the microstructure of the components by solution annealing inthe α phase area and cooling subsequently rapidly.
 2. The method asdefined in claim 1, wherein the isothermal primary shaping is carriedout by forging or extrusion molding in the α+γ phase area attemperatures ranging from 1000° C. to 1,340° C.
 3. The method as definedin claim 1, wherein the isothermal primary shaping is carried out byforging or extrusion molding in the α phase area at temperatures between1340° C. and 1,360° C.
 4. The method as defined in claim 1, wherein theisothermal secondary shaping is carried out in the α+γ phase area attemperatures ranging from 1000° C. to 1,340° C.
 5. The method as definedin claim 1, wherein the secondary shaping process and the solutionannealing process are carried out in an inert atmosphere.
 6. The methodas defined in claim 1, wherein the cooling to a final adjustment ofstructure from the α phase area above 1340° C. takes place very rapidly,especially at a rate of 10° C./s to 20° C./s in oil.
 7. A method forproducing heavy-duty components from α+γ TiAl alloys for aircraftengines or stationary gas turbines, in that preshaping encapsulated TiAlpreforms of globular structure by isothermal primary shaping in the α+γor α phase area in a temperature region of 1000° C. to 1,340° C. byforging or extrusion molding; shaping the preforms by forging to formcomponents with a specifiable contour by at least one isothermalsecondary shaping process simultaneously with dynamic recrystallizationin the α+γ or α phase area in the temperature region of 1000° C. to1,340° C. in an inert atmosphere, said preshaping and shaping steps becarried out in a heated tool of molybdenum or graphite; adjusting themicrostructure of the components by solution annealing in the α phasearea and cooling subsequently rapidly, the isothermal primary shapingbeing carried out by forging or extrusion molding in the α+γ phase areaat temperatures ranging from 1000° C. to 1,340° C., the isothermalprimary shaping being carried out by forging or extrusion molding in theα phase area at temperatures between 1340° C. and 1,360° C., saidpreforms of a TiAl base alloy of the following composition (in atompercent) are used for the primary and second shaping: 43%–47% Al 5%–10%Nb maximum 1.0% B Maximum 0.5% C, said cooling to a final adjustment ofstructure from the α phase area above 1340° C. taking place veryrapidly, especially at a rate of 10° C./s to 20° C./s in oil.
 8. Themethod as defined in claim 7, wherein preforms of a TiAl base alloy ofthe following composition (in atom percent) are used for the primary andsecondary shaping: 43%–47% Al 5%–10% Nb maximum 1.0% B Maximum 0.5% CRemainder titanium and impurities resulting from the smelting.